A polymer electrolyte fuel cell is composed of a polymer electrolyte membrane which selectively transports protons, and a pair of catalyst electrodes (a fuel electrode and an air electrode) which sandwich the polymer electrolyte membrane. By supplying a fuel gas (containing hydrogen gas) to the fuel electrode and supplying an oxidizing gas (containing oxygen gas) to the air electrode in the fuel cell which has the above-mentioned structure, electric energy can be continuously taken out.
The polymer electrolyte membrane is composed of an electrolyte which contains a polymer ion-exchange membrane or the like, such as a sulfonic acid group-containing fluorine resin ion-exchange membrane or hydrocarbon resin ion-exchange membrane. In order for the polymer electrolyte membrane to have an ion transport function, it needs to contain a given quantity of water.
The catalyst electrode is composed of a catalyst layer that promotes a redox reaction therein and of a gas diffusion layer having both air permeability and electric conductivity. The catalyst layer is in contact with the polymer electrolyte membrane. The catalyst layer contains as a main component carbon powder having platinum metal catalyst attached. The gas diffusion layer is composed of a carbon coat layer for improving adhesion to the catalyst layer and of a base layer through which a gas supplied from an external source is allowed to diffuse to the catalyst layer. An assembly of such a polymer electrolyte membranes and a pair of catalyst electrodes (a catalyst layer, a carbon coat layer, and a gas diffusion base material layer) is called a membrane electrode assembly (hereinafter referred to as “MEA”).
For securing the MEA and for avoiding possible mixing of fuel gas and oxidizing gas supplied to the MEA, the fuel cell includes separators that sandwich the MEA.
Reaction gas channels for supplying a fuel gas to the MEA may be formed in the gas diffusion layer or other layer; however, they are typically formed in the separator's surface that contacts an MEA. The reaction gas channels are connected to a manifold hole provided in the separator. Through the reaction gas channels connected to the manifold, a reaction gas can be supplied to the catalyst electrode and a surplus non-reacted gas can be discharged. The reaction gas supplied to the reaction gas channel may be humidified beforehand.
Unit fuel cells, each consisting of an MEA and a pair of separators, are stacked on top of each other and are then sandwiched by current collectors, insulating plates and end plates to constitute a fuel cell stack, which is typically secured using fixing rods.
By supplying a hydrogen-containing fuel gas to the fuel electrode and supplying an oxygen-containing oxidizing gas to the air electrode in the fuel cell having the above-mentioned structure, the following reaction takes place whereby electric energy is generated.
First, molecules of hydrogen supplied to the fuel electrode diffuse through the fuel electrode gas diffusion layer into the catalyst layer. In the catalyst layer, each hydrogen molecule is divided into protons and electrons. Protons pass through the humidified polymer electrolyte membrane toward the air electrode. Electrons move to the air electrode through the separator contacting the MEA. The electrons moving from the fuel electrode to the air electrode may be recovered as electric energy. In the air electrode catalyst layer, the protons that came from the polymer electrolyte membrane, the electrons that came from the separator, and oxygen supplied to the air electrode react together to produce water. Water produced during power generation is used for humidifying the MEA so as to prevent membrane degradation in the MEA due to drying.
Since the reaction entails heat generation, the fuel cell generates heat during power generation. Thus, the fuel cell needs to be cooled by a coolant or the like in order to keep suitable temperature for the fuel cell. In general, in a fuel cell stack in which fuel cells are stacked on top of each other, coolant channels are provided for every 1 to 3 cells.
For improved power generation performance, the flow rate of reaction gas needs to be uniform among all reaction gas channels. By equally supplying a reaction gas to reaction gas channels, the rate of reaction that takes place in the catalyst layer becomes uniform, reducing the likelihood of variations in the current distribution and improving power generation performance.
Fuel cells are known in which separators have a gas distribution section that connects a reaction gas manifold hole and reaction gas channels in order to equally feed a reaction gas to reaction gas channels (see Patent Literatures 1 to 6).
FIG. 1 is a perspective view of gas distribution section 32 of a separator in a fuel cell disclosed by Patent Literature 1. As illustrated in FIG. 1, the separator of Patent Literature 1 includes gas distribution section 32 which connects reaction gas supply manifold hole 20 and region 11 in which reaction gas channels are formed. Gas distribution section 32 includes a plurality of convexes 31.
When gas distribution section 32, which connects reaction gas supply manifold hole 20 and region 11, has a plurality of protrusions as described above, the flow of reaction gas fed through reaction gas supply manifold hole 20 into the gas distribution section is divided into multiple streams by the protrusions, whereby the reaction gas may be equally distributed to reaction gas channels.
Techniques are also known in which a frame is used that covers the periphery of an MEA and in which a gas diffusion section is formed exclusively between the frame and the separator so as not to overlap the MEA's catalyst electrode (see Patent Literatures 7 and 8).